Aeroplane Construction and Operation

by John B. Rathbun

Ordinarily, the wing is turned upside down for covering, with the concave side uppermost. The seams are sewed together so that the completed fabric is wider than the length of the wing and is a little longer than is necessary to wrap entirely around the width of the wing. The fabric is then temporarily fastened along the trailing edge, is passed under the wing to the front edge, and over the concave upper side back to the trailing edge. At this point the excess material will hang down over the rear edge. With the wing in its upside down position, the convex side will be at the bottom, and if a weight is hung on the overhanging material at the rear edge, the cloth will be pulled tight against the lower convex side and straight across and above the concave side. The fabric at the top is then stretched along the cordal line of the ribs. By laying a narrow board on top of the fabric, and near the entering edge, the fabric can be brought down uniformly along the concave edge of the ribs, and by tacking or sewing as the board is moved back the concave face can be covered without further trouble. After the concave face is disposed of, the wing can be turned over and the fabric is then fastened to the convex side of the ribs.

Fig. 11. Method of Stretching Fabric on Wings. Fabric Passes Under and Then Over Concave Side and Is Pressed Down into Hollow by a Board as Shown.

One method of fastening the linen is to lay tape over the ribs, and then drive tacks through the tape and fabric into the rib. The tape keeps the tacks from tearing through the linen. The tape should be heavy linen of from 3/4 to 1 1/4 inches wide, and laid in cellulose before tacking, so that the tape will be cemented to the fabric and the solution will be driven into the tack holes. After the tape is in place, it should be covered with not less than three coats of cellulose dope before the main surface is treated. This gives an additional three coats over the tape where it is most needed for protection against moisture. In any case, the seam or tacking strip should be pressed down so that it projects as little as possible above the general surface of the wing.

Tacking is not desirable, for tacks and nails always tend to split the thin members of the rib, and very often corrode the cloth and weaken the fabric. This has resulted in the whole fabric being ripped off while in flight. Iron or steel tacks should never be used, as they destroy the strength of the fabric very rapidly by the formation of rust, and particularly with sea-planes used on salt water. Sewing the fabric to the ribs with linen thread is the most satisfactory method and is in general use among high-grade builders.

The fabric can be stitched to bands of thread or tape, the latter being wrapped about the rib. Stitches can also be taken from one surface straight through to the other. The thread or tape bands on the ribs are merely wrappings taken around the rib flange, and through the lightening holes, these bands usually being about 4 inches apart. They at once tie the flange to the web and form a soft surface into which to take the stitches. Fig. 4 in Chapter X shows the thread bands (d) wrapped around the rib flange (G), and through the lightening hole, the fabric lying above and below the rib flanges as shown. A section view through the rib and fabric is shown at the right.

Varnishing. When varnish is to be used over the dope, only the best grade of spar varnish should be used, since any other kind is soon destroyed by moisture. From two to three varnish coats will be sufficient, and each coat should be thoroughly dried and sandpapered before the next is applied, care being taken not to injure the fabric with the sandpaper. Sandpapering between the dope layers is not necessary, since each successive coat partially dissolves the preceding coat, and thus welds the layers together. Varnish, however, does not act in this way, and the coats must be roughened. Shellac rots the linen and should not be used.

The Government specifies one coat of flexible white enamel in which a small quantity of lead chromate is mixed. This is applied over the last coat of varnish. The lead chromate filters out the actinic rays of the sun, thus reducing the injurious effect of the sunlight on the covering. If coloring matter is added to the dope, it should be in liquid form, as powders destroy the strength and texture of the dope deposit.

Fig. 12. Wing Structure of Handley-Page Giant Biplane. Courtesy "Aerial Age."